Error "if" in (tag - filename)

"$if(%artist%=="Various",%title%,%title% - %artist%)" in (tag - filename) does not work.

For which program should this syntax work?

See the documentation for the scripting functions in MP3tag:

You would have to use a syntax like this:
$if($eql(%artist%,Various),%title%,%title% - %artist%)

Your == and enclosing a value in double quotes is not a valid syntax for Mp3tag.
Where did you find this example?